Oligodynamic metals, such as silver, are effective in minute quantities as bacteriostats and bactericides. The most active form of these oligodynamic metals is as ions in solution. While the precise nature of the bactericidal effect is unknown, it is believed to involve altering the function of the cell membrane or linking to the cell's DNA to disrupt cell function. The bactericidal action is effective against a broad spectrum of bacteria, including all of the common strains which cause infection. When these metals are used in the minute concentrations required to kill or stem the growth of bacteria, they do not have any detrimental effect on normal mammalian cells.
Silver is used routinely in antibacterial salves, such as silver sulfadiazine, and has also been used in clinical trials to coat gauze for burn dressings. Medical devices, such as catheters, with silver impregnated in a soluble collagen or polymer coating are also known. After these catheters are placed, the coating slowly dissolves and the silver is released over time into the environment. The infection rates with these products are reported to be two to four times lower than standard catheters.
One catheter that uses silver as an antibacterial agent has had only limited success because the device, consisting of a silver impregnated collagen cuff which is inserted just below the skin, is difficult to place correctly. The cuff is also expensive, increasing the cost of a central venous catheter almost three-fold. Other catheters for reducing infection rates use well known approaches, most of them varying only in the type and solubility of the silver or silver-alloy coating.
Many of the prior art catheters that use oligodynamic metals as bacteriostats fail to adequately prevent infection for one or more of the following reasons: 1) Silver released from soluble coatings, is not always in the same charge state and often is not charged at all, therefore its bactericidal potential is not optimized; 2) With soluble-coated catheters, once the coating dissolves, usually over about two weeks there is no further antibacterial protection; 3) A non-soluble silver, silver alloy or silver-oxide coating can prevent colonization of the catheter to a limited extent, but the oligodynamic metal is not released into the surrounding fluid or tissue; 4) Due to the substantial change in the catheter placement procedure, the use of these catheters requires additional personnel training; and 5) Although infection can enter the body through either the interior or the exterior of the catheter, not all catheters provide both interior and exterior protection. Furthermore, despite the capability of silver-alloy coated devices to produce a two to four fold reduction in bacterial colonization, their high cost greatly detracts from their modest capabilities.
Research from the 1970's onward has been directed toward improving the antibacterial effects of oligodynamic metals by electrically injecting the metal ions into solution. This process, known as oligodynamic iontophoresis, is capable of reducing bacterial colonization fifteen to one-hundred fold. Iontophoresis describes the movement of ions in a conductive fluid under the influence of low-strength electric fields, and in this context refers to the forcing of ions into a conductive fluid environment using minute electric currents. For example, if two electrodes made of a metal, such as silver, are introduced into a conductive medium, such as saline, blood or urine, and an electrical potential is applied across the electrodes, silver ions are driven into solution creating an enhanced bactericidal effect. The current required to safely drive a sufficient amount of silver ions into solution to control infection is in the range of 1 to 400 microAmperes. This current range does not cause localized cell necrosis and it is below the sensory or pain threshold.
Despite its great potential, the oligodynamic iontophoresis phenomenon has found limited use in conjunction with medical devices, although urological or Foley catheters have progressed to animal experiments. With respect to Foley catheters, researchers have identified several deficiencies in prior art devices. Foremost is that the electrodes used to force ions into solution wear out, or corrode, at the interface between air and the conductive medium. This problem probably also arises in blood or saline environments as well as urine. Other significant drawbacks with prior art iontophoretic devices include bulky, current controlled power sources required for driving the electrodes; electrode configurations that do not protect both the outside and the inside of the catheter; and manufacturing processes that are labor intensive.
An example of an infection control catheter that uses separate electrodes on the catheter and an external power supply to drive ions into solution is U.S. Pat. No. 4,411,648 to Davis. Other prior art oligodynamic iontophoresis devices do not use external power supplies. For example, U.S. Pat. No. 4,886,505 to Haynes, teaches placing two metals in direct physical contact to produce electrical currents. The currents produced, however, are likely to be too large to be safely used and possibly will alter the pH of the environment. In German Patent Document DE 3,830,359, two dissimilar metal powders not in electrical contact with each other are embedded in a nonconductive catheter material, such as electrically insulating polymers. Because of the separation of dissimilar metals by an insulator, it is not likely that there is any iontophoresis effect in this device as a result of a potential being created by the dissimilar metals, except for the possibility of when a biofilm forms on the catheter surface to complete the circuit. Were an electrical circuit to be formed in this manner, the current density would not be regulated or predictable, and the current produced therefore could be either too high to be safe or too low to be effective.
An oligodynamic iontophoresis catheter which uses the properties of metals to generate a current and to form an ion barrier for killing bacteria at a localized body entry is disclosed in U.S. Pat. No. 4,569,673 to Tesi. Tesi teaches placing a strip of an oligodynamic metal on a nonconductive substrate. The oligodynamic metal acts as a sacrificial galvanic anode and gives off ions when placed in conductive contact with a dissimilar metal by placing the catheter in an electrolytic solution. Because the conductivity and pH of urine, for example, varies over time within the same person, as well as from individual to individual, it would be extremely difficult to achieve a specific current density at a given time with any precision or predictability. Additionally, the Tesi device only provides localized infection control.
Thus, none of these devices fulfill the promise held out by oligodynamic iontophoresis for reducing infection in long-term indwelling medical devices.